This invention belongs to the technical filed of flexible wiring boards. More particularly, it relates to a technique of constructing flexible wiring boards having wiring films in which a high electric current appropriately flows.
There have been frequently employed flexible wiring boards having wiring films patterned in desired.
In FIG. 12, the numerical symbol 110 stands for an example of flexible wiring boards of the conventional art.
This flexible wiring board 110 has a first resin film 117, first and second wiring films 121 and 122 located on the first resin film 117, and a second resin film 125 formed on the first and second wiring films 121 and 122.
Although the first and second wiring films 121 and 122 has the same thickness, the first wiring film 121 is wider than the second wiring film 122. Therefore, the sectional area of the first wiring film 121 is larger than the sectional area of the second wiring film 122.
Since the first wiring film 121 having the larger sectional area has a smaller electrical resistance than the second wiring film 122, a higher current can flows through it.
With the recent tendency toward high-density flexible wiring boards, it is required to be high density wiring film. However, such wide first wiring film 121 as described above interferes the density of wiring film.
The present invention, which has been made to solve the troubles encountering in the conventional art as described above, aims at providing a technique of constructing flexible wiring boards having wiring films which show a small electrical resistance and yet have been finely patterned.
To solve the above-described problems, the present invention provides a flexible wiring board having a first resin film, a first wiring film the bottom face of which is embedded into the first resin film, and a second wiring film the bottom face of which is adhered to the surface of the first resin film.
The present invention further provides a wiring board wherein the surfaces of the first wiring films is flush with second wiring.
The present invention further provide a flexible wiring board wherein a second resin film is formed on the surfaces of the first and second wiring films.
The present invention further provides a flexible wiring board wherein at least one first opening is formed in the part of the second resin film where the first wiring film is disposed.
The present invention further provides a flexible wiring board wherein at least one second opening is formed in the part of the second resin film where the second wiring film is disposed.
The present invention further provides a flexible wiring board wherein at least one third opening is formed in the part of the first resin film where the first wiring film is disposed.
The present invention further provides a flexible wiring board wherein a metal bump is located in at least one of the first openings.
The present invention further provides a flexible wiring board wherein a metal bump is located in at least one of the second openings.
The present invention further provides a flexible wiring board wherein a metal bump is located in at least one of the third openings.
The present invention further provides a process for producing a flexible wiring board which comprises the step of etching a metal foil halfway in the thickness direction to form concavities of a predetermined pattern on the surface of the metal foil, the step of forming a first resin film on the surface of the metal foil in the side where the concavities are formed, the step of forming a resist layer, which is provided with openings at the parts corresponding to the concavities, on the back face of the metal foil, and the step of etching the parts provided with the openings of the metal foil so as to divide the first wiring film having the same thickness as the thickness of the part of the metal foil where no concavity is formed and the second wiring film having the same thickness as the thickness of the part where the concavities are formed.
The present invention further provides a process for producing a flexible wiring board wherein a second resin film is formed on the first resin film in the side where the first and second wiring films are located.
The present invention further provides a process for producing a flexible wiring board wherein an opening is formed in the part of the first resin film where the first wiring film is located.
The present invention further provides a process for producing a flexible wiring board wherein openings are formed either in one or both of the part of the second resin film where the first wiring film is located and the part where the second wiring film is located.
The present invention has the constitution as described above. The flexible wiring board according to the present invention has the first and second wiring films separated from each other by a groove. The first wiring film is embedded into the first resin film and, therefore, has a thickness larger by the embedded depth than the film thickness of the second wiring film. Since the section area of the first wiring film is larger because of the larger film thickness, the electrical resistance of the first wiring film can be lowered even in case where the first wiring film has the same width as the second wiring film.
In the process for producing the flexible wiring board according to the present invention, concavities of a predetermined pattern are formed in a metal foil and then a resin solution is applied to the face having these concavities, thereby filling the concavities with the resin solution. Subsequently, the whole construct is dried and baked. Thus a first resin film having a flat surface can be formed.
When the above-described metal foil is patterned and a resin solution is applied to the surface of each wiring films thus formed, grooves located among these wiring films are filled with the resin solution. After drying and baking, a second resin film having a flat surface can be formed.
The surfaces of the first is flush with the second wiring film. When the above-described second resin film is formed on the first and second wiring films, the thickness of the second resin film on the first wiring film is almost the same as the thickness of the second resin film on the second wiring film. By forming openings in the second resin film by etching, therefore, the surfaces of the first and second wiring films can be exposed at the bottom faces of the openings almost at the same time as the etching proceeds.